Vehicular repeaters operate to allow a portable unit to communicate via the vehicular repeater with a base unit. This arrangement increases the "talk-back" range of the portable by relaying upon the higher power transmitter of the vehicular repeater. Generally, however, vehicular repeater systems have an inherent detriment when communication is directed from the base through the vehicular repeater to the portable. This detriment stems from the simplex operation of the vehicular repeater. Typically, base units cause the vehicular repeater to be held in the transmitting-to-portable mode for a brief period at the end of each transmission. Accordingly, the vehicular repeater cannot receive a response transmission from the portable immediately following the conclusion of a voice message from the base due to this "hang time". Also, a portable operator may desire to have access at some time during a transmission for the vehicular repeater to transmit, for example, an emergency message. Thus, it is generally desirable to permit the portable to have priority since the portable operator is likely to be in a dangerous situation (for example a police officer or fireman). However, due to simplex operation, if the vehicular repeater is transmitting, it will not respond to transmissions from the portable.
To solve this access problem, some vehicular repeater designers have opted to create small"holes" (i.e., interruptions) in the repeated audio to the portable to permit the vehicular repeater to switch to its receiving mode in an attempt to detect a transmitted carrier from the portable. In this way, if the portable should transmit during a message from the base the portable transmission may be given priority (if its transmitter carrier is detected) and be permitted to take control of the vehicular repeater.
While generally effective, contemporary vehicular repeater systems suffer from two detriments which are typically balanced against one another in any particular implementation. The first detriment is that during the interruptions in the repeated audio (during which time a vehicular repeater looks for the transmitted carrier from the portable), an unsquelched noise burst is apparent in the speaker of the portable. The second detriment stems from the recognition of those skilled in the art that carrier detection circuits are prone to falsing. Since these noise burst are generally undesirable, and may impair the proper reception of the message, it is desirable to keep the interruptions as brief as possible. However, a shorter detection interval reduces the time that the carrier detection circuit has to examine the incoming signal, and therefore, increases the likehood of falsing. Additionally, falsing may occur in situations where several portables are clustered in a small area and are talking on the same frequency. A typical situation comprises police or fire scenes, which may involve several individuals using portable radios. In these situations, stray signals from portables on the same frequency may be received by a vehicular repeater other than the one actually associated with the transmitting portable. These falsing errors cause an improper seizure of the vehicular repeater which interrupts the message from the base and prevents reception by the portable.
Accordingly, there is a need in the art for a vehicular repeater system that minimizes or eliminates the falsing and noise burst detriments of the prior art thereby providing improved system and portable performance.